Method for the plasma cleaning of the surface of a material coated with an organic substance and the installation for carrying out said method

ABSTRACT

The invention relates to a method of cleaning the surface of a material ( 4 ) that is coated with an organic substance. The inventive method is characterised in that it comprises the following steps, consisting in: introducing the material ( 4 ) into a treatment chamber ( 2 ), having a pressure of between 10 mbar and 1 bar therein, which is supplied with a gas stream containing at least 90 volume percent of oxygen; and generating a plasma by passing an electric discharge between the surface of the material and a dielectric-covered electrode ( 5   a,    5   b,    5   c,    5   d,    5   e,    5   f,    5   g ) in order to break down the organic substance under the action of the free radicals O thus produced. The invention also relates to an installation ( 1 ) that is used to carry out said method.

BACKGROUND OF THE INVENTION

The present invention relates to a process for cleaning, by means of aplasma at a pressure between 10 mbar and 1 bar, the surface of amaterial coated with an organic substance, and to an installation forimplementing this process, more particularly one intended for cleaningmetal strip.

Within the context of the present application, the expression “organicsubstance” is understood to mean any water-insoluble compound containingcarbon, oxygen and hydrogen.

DESCRIPTION OF THE PRIOR ART

Strip coming from the various manufacturing lines are generally coveredwith an oil film which may have come from two sources. Firstly, thisfilm may have been applied by spraying protective oil, so as to protectthe surface of the strip from corrosion. However, it may also derivefrom a residual oil film in the case of strip coming from a cold rollingmill or skin-pass. In both cases, the oil coating weights may be up toseveral hundred mg per m².

To deposit a metal or organic coating on such strip requires the removalof the oil film during a degreasing operation, in order to obtain goodadhesion of this coating. The techniques generally employed for thispurpose on industrial lines must not overheat the strip so as topreserve the mechanical properties of the steel strip.

Thus, the most common of these techniques consists of an alkalinedegreasing operation which may or may not be assisted by an electrolyticprocess. For environmental reasons, this process requires theinstallation of complex ancillary workshops for the reprocessing ofeco-toxic co-products.

Other technical solutions are used to prevent the formation of suchco-products, such as for example laser ablation, which has the effect ofdesorbing the organic compounds photochemically, but does not allowstrips to be treated at speeds exceeding 10 m/min for lack of power.

It has recently been discovered that an advantageous cleaning techniqueconsists in using a plasma at a pressure close to atmospheric pressure,produced by means of dielectric barrier discharges in oxygen-containinggas mixtures. A reaction then occurs between the reactive oxygen species(O., etc.) that are formed and the organic compounds of the oil, withproduction of carbon dioxide and water.

A dielectric barrier discharge has the advantage in particular ofgenerating a cold plasma, which does not degrade the characteristics ofthe strip.

However, to obtain a stable and homogeneous discharge at pressures closeto atmospheric pressure generally requires having a mixture consistingvery predominantly of helium. The proportion of oxygen in the mixture istherefore low, and it is found that the treatment is not sufficientlyrapid, probably because of the low density of reactive oxygenatedspecies, but also because of inopportune polymerization of the organicsubstance to be removed.

Thus, U.S. Pat. No. 5,529,631 describes the treatment of plasticsrunning through by a cold atmospheric-pressure plasma. The dischargesare stabilized in gas mixtures based on helium with optionally up to 25%by volume of another gas. The technique requires strict control of theatmosphere in the plasma chamber by fitting an airlock at the inlet andoutlet of the vessel. The use of helium as plasma gas and the complexityof the equipment makes this process as expensive and as difficult toimplement as a conventional vacuum process. Furthermore, it does notallow degreasing of a strip running at a speed of greater than 3 m/min.

Moreover, U.S. Pat. No. 5,938,854 describes a process for cleaningplastic and metallic surfaces by a homogeneous glow discharge initiatedin air at pressures between 10 torr and 20 bar. In addition to complexequipment, working at these pressure in air requires the dischargestriking voltage, which is directly proportional to the pressure, to beconsiderably increased.

SUMMARY OF THE INVENTION

The object of the present invention is therefore to make available aprocess for cleaning the surface of a material coated with an organicsubstance that makes it possible to obtain uniform cleaning of saidsurface at a treatment rate of at least 10 m/min and at pressures closeto atmospheric pressure.

For this purpose, a first subject of the invention consists of a processfor cleaning the surface of a material coated with an organic substance,comprising the steps consisting in:

-   -   introducing said material into a treatment chamber, having a        pressure of between 10 mbar and 1 bar therein, which is supplied        with a gas stream containing at least 90% oxygen by volume; and    -   generating a plasma by passing an electric discharge between the        surface of said material and a dielectric-covered electrode, so        as to break down said organic substance through the action of        the free radicals O. thus produced.

The present inventors have found that this process allows the substrateto be treated uniformly and rapidly, although the discharge obtained inthis gas mixture consisting predominantly of oxygen is not homogeneous.The discharge mode seems to lie between filamentary discharge and coldarc. This is because the uncharged active species O. generated by theplasma are distributed over the surface of the strip through the actionof the flux and independently of the electric field, and uniformlyremove the material coated with organic substance, owing to theirincreased density due to the high proportion of oxygen that is present.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

In a preferred method of implementation, the oxygen and/or ozonemolecules that are formed by recombination of the free radicals O.produced in said plasma are re-dissociated. It is thus possible toincrease the density of uncharged active species that are distributedover the surface of the strip independently of the electric field,further improving the uniformity of the treatment.

This re-dissociation may be carried out by means of UV radiation ofsuitable wavelength, which allows the ozone produced by recombinationaway from the cold arcs to dissociate into molecular oxygen and into theradical O.

In another preferred method of implementation, a sinusoidal voltage, thefrequency of which is between 10 and 100 kHz, is applied in order toinitiate the discharge. Indeed, this type of voltage results in thealmost continuous presence of active species in the inter-electrodespace, thereby making it possible to achieve high kinetic yields.

In another preferred method of implementation, the energy dissipation inthe discharge is less than 40 W/cm² and the voltage applied in order toinitiate the discharge is less than 4.4 kV.

The present inventors have in fact found that the inhibiting effects dueto the polymerization of the oil are greater the higher the appliedvoltage and that the surface treatment is nonuniform. This is becausethe oxidation and elimination of the oil take place essentially at thepoint where the discharges impact on the strip, whereas the oilpolymerizes away from these more intense glow channels. Increasing thevoltage across the terminals of the discharge results in an increase inthe energy of the electrons that all the more easily initiate thepolymerization of the oil.

The process according to the invention may furthermore have thefollowing features, individually or in combination:

-   -   the voltage applied in order to initiate the discharge is        sinusoidal;    -   the material is in the form of a running strip and the various        steps of the process are carried out continuously by means of        installations placed in succession along the path of the running        strip;    -   one of the faces of the material is treated followed by the        other;    -   the material to be treated is a metallic material, preferably a        carbon steel; and    -   the process is carried out in order to degrease the surface of        metallic materials, prior to a coating being deposited on this        surface.

A second subject of the invention consists of an installation whichcomprises at least one module comprising a treatment chamber, means forsetting the pressure inside said chamber to a value between 10 mbar and1 bar, means for making said grounded strip run through said chamber, aseries of dielectric-covered electrodes placed so as to face thatsurface of said strip to be treated, these electrodes being connected toa sinusoidal high-voltage generator, means for supplying said chamberwith gas and means for extracting from said chamber the gases resultingfrom the decomposition of the organic substance coating the strip.

In a preferred embodiment, said installation comprises a succession ofan even number of said modules, through which said strip runs insuccession, exposing in turn one of its faces to the electrodes of saidmodules.

In another preferred embodiment, said installation furthermore includesUV emission lamps placed between said electrodes.

DESCRIPTION OF THE DRAWINGS

The invention will be illustrated by the description of two embodimentsgiven by way of indication, but implying no limitation, with referenceto the appended drawings in which:

FIG. 1 shows a schematic view of a treatment installation according tothe invention;

FIG. 2 shows a schematic view of a treatment installation according tothe invention for a successive treatment of the two faces of a materialin the form of a running strip;

FIGS. 3A and 3B show two images of surfaces of strip treated in thepresence of UV irradiation (FIG. 3B) and without said irradiation (FIG.3A);

FIG. 4 demonstrates the increase in density of the oxygen radicalsduring the additional application of UV radiation at 253 nm;

FIG. 5 shows the dependence of the oxygen radical density as a functionof the intensity I of the current applied in the discharge;

FIG. 6 shows the change in protective oil coating weight present on astrip as a function of the electron dose It/S which is applied to it;and

FIG. 7 shows the Auger electron spectrum of the surface of a stripdegreased using a discharge involving an electron dose of 21 mC/cm².

FIG. 1 shows a diagram of an installation according to the invention forimplementing the process according to the invention, in order to treat ametal strip made for example of carbon steel. This installationcomprises a module 1 consisting of a treatment chamber 2 in which thereis a cooled roll 3 around which the metal strip 4 runs. The roll 3 andthe strip 4 are grounded. Pumps (not shown) allow the pressure insidethis chamber 2 to be set to a value between 10 mbar and 1 bar.Electrodes 5 a, 5 b, 5 c, 5 d, 5 e, 5 f and 5 g covered with adielectric (alumina) are placed so as to face the strip 4. Theseelectrodes 5 a, 5 b, 5 c, 5 d, 5 e, 5 f and 5 g are connected to thehigh voltage supplied by a medium-frequency sinusoidal generator 6 (thefrequencies being between 10 and 100 kHz). The high-voltage electrodes 5a, 5 b, 5 c, 5 d, 5 e, 5 f and 5 g are also cooled. In order to optimizethe energy invested in the discharge, the fixing of the high-voltageelectrodes 5 a, 5 b, 5 c, 5 d, 5 e, 5 f and 5 g allows theinter-electrode distance to be varied.

The module also includes means for supplying said chamber with gas, andmeans for extracting from said chamber the gases resulting from thedecomposition of the organic substance coating the strip 4 (these meansnot being shown).

In this particular embodiment, UV lamps 7 a, 7 b, 7 c, 7 d, 7 e and 7 fare placed between the high-voltage electrodes 5 a, 5 b, 5 c, 5 d, 5 e,5 f and 5 g so as, on the one hand, to allow the treatment at thedischarge to be made uniform and, on the other hand, to allow the ozone,which is formed away from the inter-electrode volume, to dissociate.Consequently, the strip 4 may continue to be degreased away from theinter-electrode space by the O. radicals coming from the dissociativeabsorption of the ozone induced by applying the additional UV (253 nm)radiation.

FIG. 2 is a schematic representation of an installation according to theinvention that comprises a succession of four modules 10, 11, 12 and 13for carrying out a successive treatment of both faces of a running strip14. The four modules 10, 11, 12 and 13 are connected together viaintermediate components housing the pumping set and the gas injectionsystem, which ensures that the installation is exposed to the stream andtherefore ensures that the treatment is uniform, despite thecharacteristics of the inhomogeneous discharge.

EXAMPLES

Trials were carried out on small-sized (20 to 25 cm²) strips in staticmode, these being coated with a protective oil (Tinnol 200® from QuakerChemical) as it was necessary to complete the degrease, in order tosimulate a cleaning treatment before coating.

Apparatus Used

The trials were carried out in a dielectric barrier reactor consistingof an electrode covered with a 0.7 mm layer of alumina and of a groundedmetal electrode over which the strip to be treated is placed. Thealumina-covered electrode was connected to the high voltage (350 to 4400V). The high voltage was delivered by a medium-frequency (3 to 30 kHz)sinusoidal generator. The two electrodes were equipped with a coolingsystem allowing them to remain at temperatures close to ambienttemperature during operation of the plasma.

The inter-electrode distance may be set between one millimeter andseveral tens of millimeters.

Example 1

Two identical carbon steel strips, coated with a 186 mg/m² protectiveoil layer were treated. The other parameters were identical for the twotreatments, namely:

-   -   200 mbar of oxygen;    -   12 kHz sinusoidal voltage; 3.6 kV; current: 30 mA;    -   inter-electrode distance: 5 mm.

The strip treatments illustrated in FIGS. 3A and 3B differ only by UVradiation being imposed in one case and not in the other.

FIG. 3 shows images of the surface of strips treated by a dischargeinitiated in just oxygen, with (FIG. 3B) and without (FIG. 3A)additional UV (253 nm) irradiation. The dark areas correspond to thenon-greased regions where the oil has polymerized.

It may be seen that applying UV radiation in addition to the dischargeresults in less polymerization of the oil, thus allowing bettercleaning, in a shorter time.

Applying UV radiation whose wavelength corresponds to the dissociativeabsorption of ozone results in the uniform presence of oxygen radicalson the surface of the strip that allow cold combustion of the oil.

Applying UV radiation in addition to the discharge results not only in amore uniform distribution of the oxygen radicals on the surface of thestrip, but it also increases the density of the O radicals, all thedischarge parameters (voltage, frequency of application of the voltage,current, pressure and inter-electrode distance) remaining constant.

FIG. 4 demonstrates the increase in density of the oxygen radicalsduring application of 253 nm UV radiation by means of optical emissionspectroscopy (OES). The emission wavelength of the excited oxygenradicals lies at about 777 nm. This figure shows the intensity I₇₇₇ ofthe radiation at 777 nm as a function of time t. The various regions ofthe graph correspond to the following phases:

-   -   zone A: no electrical discharge or UV radiation applied. The        recorded intensity corresponds to the background noise;    -   zone B: an electric discharge has been applied in pure oxygen,        without UV radiation being applied;    -   zone C: in addition to the electric discharge, UV radiation at        253 nm has been applied;    -   zone D: the UV radiation is maintained, but in the absence of        electrical discharge; and    -   zone E: the UV radiation is stopped and the background noise        returns.

Example 2

FIG. 5 shows, using optical emission spectroscopy, that the density ofactive oxygenated species O. varies linearly with the intensity of thecurrent applied in the discharge.

The discharge currents plotted in this figure were varied both atconstant voltage, by varying the rate of application of the V, andtherefore varying the impedance of the dielectric, and at constantfrequency by varying the voltage. This FIG. 5 therefore shows that thedensity of active species depends only on the intensity of the dischargecurrent and is not in any way influenced by the discharge voltage atconstant current. This means that it is possible to obtain the samedensity of active species at power levels that differ only by theimposed voltage, the current remaining constant. However, it has beenfound that too high a voltage results in polymerization of the oil,which tends to inhibit the rate of oxidation of the organic residuespresent on the surface of the strip. In addition, industrial applicationrequires the dissipation of a minimal energy density (less than 40W/cm²·s) in the discharge. Consequently, the discharge conditions neededfor degreasing a strip are achieved by maximizing the current for aminimal imposed voltage.

The influence of the power of the discharge at constant current on therate of degreasing is demonstrated in the table below, which combinestwo tests carried out by varying the frequency of the applied sinusoidalcurrent:

Discharge Percentage Frequency power (W) Voltage (V) degreasing (kHz)110 3400 83% 10 55 1720 87% 20

It may therefore be seen that, at equal discharge current, and for anidentical treatment time, the efficiency of the degreasing is better atlower voltage and therefore at lower power.

Example 3

A 20 cm² strip covered with 186 mg/m² of protective oil was treated bythe process according to the invention. In the present case, thedischarge was initiated in a stream of oxygen at a pressure of 350 mbar.The oxygen and/or ozone molecules formed from the recombined freeradicals O. were not re-dissociated. FIG. 6 shows the change in coatingweight of protective oil present on the strip as a function of theelectron dose It/S (i.e. the electron current density multiplied by thetreatment time). Application of the stream allows uniform treatment ofthe strip, which was confirmed by grazing-incidence infrared absorptionspectroscopy (IRRAS).

FIG. 7 shows the Auger electron spectrum of the surface of the stripdegreased using a discharge involving an electron dose of 21 mC/cm².Only the iron and oxygen peaks are present. The absence of a carbon peakaround 273 eV confirms that the strip was completely degreased.

1. A process for cleaning the surface of a material coated with anorganic substance, the process comprising: introducing said materialinto a treatment chamber, having a pressure of between 10 mbar and 1 bartherein, which is supplied with a gas stream containing at least 90%oxygen by volume; and generating a plasma by passing an electricdischarge between the surface of said material and a dielectric-coveredelectrode, breaking down said organic substance through the action ofthe free radicals O^(.) thus produced, wherein said material is a carbonsteel.
 2. The process as claimed in claim 1, wherein the oxygen and/orozone molecules that are formed by recombination of the free radicalsO^(.) produced in said plasma are re-dissociated.
 3. The process asclaimed in claim 1, wherein said re-dissociation is carried out using UVradiation of a suitable wavelength.
 4. The process as claimed in claim1, wherein the voltage applied to initiate the discharge is sinusoidaland has a frequency of between 10 and 100 kHz.
 5. The process as claimedin claim 1, wherein the energy dissipation in the discharge is less than40 W/cm² and the voltage applied in order to initiate the discharge isless than 4.4 W.
 6. The process as claimed in claim 1, wherein thematerial is in the form of a running strip and said introducing andgenerating operations are carried out continuously using installationsplaced in succession along the path of the running strip.
 7. The processas claimed in claim 6, wherein one of the faces of said strip is treatedfollowed in succession by the other.
 8. The process as claimed in claim1, wherein the process degreases the surface of said metallic materials,and further comprises depositing a coating after said degreasing.
 9. Theprocess as claimed in claim 1, further comprising removing oil from thesurface of metallic materials using the generated plasma.
 10. Theprocess as claimed in claim 3, wherein suitable wavelength isapproximately 253 nm.